A derogatory word used in name calling. A combination of the word struggleand the traditional def of bucket - "a typically cylindrical vessel for catching, holding, or carrying liquids or solids".
By combining these words your are implying that not only does the person being verbally attacked struggle, but they do it so much or so well, they could fill an entire bucket full of it.
John Doe - "dude, you've been trying to do that for like and hour. Let me do it!"